With the advent of integrated electronics, electrical circuits have largely eliminated mechanical and electromechanical features in many applications, reducing the maintenance and adjustment chores associated with such applications. However, these electrical circuits as well as the mechanical aspects of the circuit housing, attachment points, and so on, may still occasionally require maintenance, adjustment, or even replacement. For example, power circuits may be exposed to excess heat and may be degraded. Vibration and other mechanical forces may affect the circuits and/or their housings or their connections to other circuit elements. However, due to the relative permanence of integrated circuit assemblies, these assemblies are not often configured for easy disassembly.
Compounding this problem, integrated circuits and other electrical assemblies are often connected to other circuitry via metallic solder because of its strength as an adhesive as well its relatively low impedance to the flow of electrical current. Solder enters a liquid state at a much lower temperature than many other metals, allowing ease of application, and remains conductive through repeated thermal cycling.
With the advent of the PCB (printed circuit board), the use of solder became widespread as a means of connecting components to PCB's. A PCB typically comprises a fiber or fiberglass board covered with metallic circuit traces, having components attached to the board in prescribed locations to contact the traces. Typically, such components have one or more “pins” or leads attached to the component that extend, parallel to one another, in one direction. During assembly, these leads are inserted through holes in the PCB (and in the overlying metallic traces) and are soldered to the traces. Often, the traces are shaped into “pads” or contacts at the points of insertion to ensure better attachment and conductivity between the component pins and the traces. PCBs made in this way tend to be cost effective, robust and reliable.
As noted above, however, electronic assemblies sometimes require disassembly, and such disassembly may require removal of one or more components from the board. It is often advantageous to affix certain assemblies to a housing or other nearby structure for purposes of heat transfer, mechanical support, and so on. However, by joining the board and housing in a semi-permanent manner, this practice can render one side of the board inaccessible, requiring desoldering of components to allow the remainder of the board to be accessed for inspection and repair.
Most solders melt at temperatures that are high enough to damage the housings and internal circuitry of many electrical components. In addition, mechanical stress and strain imposed during insertion and removal of components may weaken or damage components. Ideally, circuit components should be desoldered without damaging the components in question to the extent they are salvageable, and without causing damage to any surrounding components through the application of excess heat or mechanical strain.
Thus, desoldering and disassembly must be executed with care. At the same time, for large scale remanufacturing operations, efficiency is important, and removing and replacing multipin devices from PCBs by hand is still a slow and tedious process. Often, it is performed using a soldering iron and braided wick, or with a vacuum soldering iron to melt and remove solder. However, these methods require skill and care to prevent damage to the PCB, and the complexity and failure rate of these techniques essentially preclude efficient large-scale use.
Although a complete solution to this problem has never been found, certain attempts have been made over the years. For example, U.S. Pat. No. 4,506,820 to John Brucker describes a limited system for desoldering multipin components from printed circuit boards. According to the system of the '820 patent, a large shallow solder pot, in combination with a flexible mask, is used to apply molten solder to a selected area of a printed circuit board so as to desolder a selected device. Essentially, the '820 mask is applied to all surfaces of the board for which desoldering is not desired. In this manner, the components at the masked locations will in principle remain soldered when the entire assembly is exposed to the solder bath in the large shallow solder pot, and any unmasked components will be desoldered. However, in addition to the need to supply a pool of molten solder as large as the entire PCB, the '820 method also requires the design, creation, application, and removal of masks, with all attendant costs and complications. In addition, regardless of whether the '820 masks function properly during a given use, they do not appear to provide a robust and cost-effective solution for repetitive desoldering tasks.